US 6943119 B2
In accordance with the objectives of the invention a new Method and recipe is provided for etching of stacked layers of polysilicon. The invention provides for an added flash step after the conventional Overall Etch (OE).
1. A method for creating stacked layers of polysilicon, comprising:
providing a substrate;
patterning a first layer of polysilicon over said substrate;
depositing a second layer of polysilicon over said substrate, thereby including the patterned first layer of polysilicon; and
etching said second layer of polysilicon, thereby removing remnants of the second layer of polysilicon from sidewalls of the patterned first layer of polysilicon;
wherein said etching said second layer of polysilicon comprises a break-through etch (BT), a main etch (ME), an over etch (OE) and a flash-etch.
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6. A method for creating stacked layers of gate material for mixed-mode semiconductor devices, comprising:
providing a substrate;
creating a layer of gate oxide over said substrate;
depositing a first layer of gate material over said layer of gate oxide;
patterning and first etching said layer of first sate material;
depositing a layer of inter-polysilicon dielectric material over said substrate, thereby including exposed surfaces of said first etched layer of first gate material;
depositing a second layer of gate material over said layer of inter-polysilicon dielectric material; and
patterning and second etching said second layer of gate material;
wherein said patterning and second etching said second layer of gate material comprises the steps of a break-through etch (BT), a main etch (ME), an over etch (OE) and a flash etch.
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14. A method for creating stacked layers of polysilicon, comprising:
providing a substrate, at least one patterned first layer of polysilicon having been created over said substrate;
depositing a second layer of polysilicon over said substrate, thereby including the at least one patterned first layer of polysilicon; and
etching said second layer of polysilicon, thereby removing remnants of the second layer of polysilicon from sidewalls of the at least one patterned first layer of polysilicon, said etching said second layer of polysilicon comprising a breakthrough etch (BT), a main etch (ME), an over etch (OE) and a flash etch.
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19. A method for creating stacked layers of polysilicon for mixed-mode semiconductor devices, comprising:
providing a substrate;
creating a layer of gale oxide over a surface of said substrate;
depositing a first layer of gate material over said layer of gate oxide;
patterning and first etching said layer of first gate material;
depositing a layer of inter-polysilicon dielectric material over the surface of said substrate, thereby including exposed surfaces of said first etched layer of first gate material;
depositing a second layer of gate material over a surface of said layer of inter-polysilicon dielectric material; and
patterning and second etching said second layer of gate material, said patterning and second etching said second layer of gate material comprising a break-through etch (BT), a main etch (ME), an over etch (OE) and a flash etch step.
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1. Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of etching layers of polysilicon on a mixed-mode processing environment. The method provides for the removal of poly-2 residue from sidewalls of a poly-1 pattern while device Isat performance and mixed-mode product yield are improved.
2. Description of the Prior Art
The fabrication of semiconductor devices requires the application of numerous interdependent technical disciplines which are collectively applied in a high-speed, cost-competitive semiconductor manufacturing facility.
To enhance the competitive nature of creating semiconductor devices and to further enhance the performance characteristics of these devices, a number of semiconductor devices are of a mixed-mode design in which both analog and digital circuits are created on the same chip.
Device performance is further improved by increasing device density, since this approach reduces the interconnect and the distance between other functional elements of the device while at the same time allowing for the creation of a larger number of active devices in or on the surface of one substrate.
It is obvious that device yield is a critical parameter in creating semiconductor devices since device yield is directly related to the cost of the created devices. From this is it obvious that a continuous effort is made to increase the device yield while at the same time maintaining or improving device performance characteristics.
More specifically, in the creation of 0.35 μm mixed mode semiconductor devices, two overlying layers of polysilicon are frequently used for as an example the creation of flash memory EEPROM devices, which use a double poly structure whereby the upper poly forms the control gate and the word lines of the structure while the lower poly is the floating gate. In a typical structure, the control-gate poly overlaps the channel region that is adjacent to the channel under the floating gate. The extension of the control gate over the channel region is referred to as the series enhancement-mode transistor and is required because when the cell is erased, a positive charge remains on the floating gate inverting the channel under floating gate. The series enhancement-mode transistor prevents the flow of current from the source to the drain regions of the MOS device.
Device performance improvements are typically achieved by reducing device dimensions, which further enables increased packaging density of the created semiconductor devices. It is therefore desirable to create memory devices over smaller surface regions of the substrate. One of the frequently applied methods for the creation of etched layers of poly, that are part of a split gate flash memory device, is the use of a hardmask layer that overlies a layer of poly that needs to be etched.
The invention address concerns and aspects of creating stacked or overlying layers of polysilicon and, more specifically, concerns of poly-2 residue over sidewalls of patterned underlying poly-1.
U.S. Pat. No. 6,165,375 (Yang et al.) claims a flash step in an etch process.
U.S. Pat. No. 6,165,861 (Liu et al.) shows a method for a mixed mode product.
U.S. Pat. No. 6,103,622 (Huang) and U.S. Pat. No. 6,103,621 (Huang) show processes for mixed mode products using poly etches.
A principal objective of the invention is to provide a method for the creation of stacker layers of polysilicon such that no upper layer residue is present on sidewalls of a patterned lower layer of polysilicon.
Another objective of the invention is to provide a method of creating stacked layers of polysilicon such that the Isat performance of semiconductor devices that are created with the stacked layers of polysilicon is enhanced.
Yet another objective of the invention is enhance yield of mixed-mode created semiconductor devices that comprise stacked patterned layers of polysilicon.
A still further objective of the invention is to remove low Isat device performance as a yield detractor for mixed-mode semiconductor devices.
In accordance with the objectives of the invention a new method and recipe is provided for etching of stacked layers of polysilicon. The invention provides for an (added) flash step after the conventional Overall Etch (OE). This flash step is combined with applying zero bias power and high source power, which combination results in the complete removal of poly-2 residue even for high P.D. mixed-mode devices. The flash step further desirably increase Isat from 0.2 mA to 0.6 mA, thus removing the Isat parameter as a yield detractor and making the yield independent of the device parameters (Critical Dimension or CD) and of the thickness of inter-poly oxide (IPO).
Conventionally, in the creation of mixed-mode semiconductor devices, low Isat performance of the created devices that comprise stacked layers of patterned polysilicon, has been a random but significant yield detractor.
It has been found, an observation which increases in its severity of impact if the device density increases, that the patterned layer of polysilicon-2 has a high topography, which makes a satisfactory patterning of the layer of polysilicon-2 difficult the control. The poly-2 is etched before subsequent steps of Lightly Doped Diffusion (LDD) source/drain impurity implantations into the substrate.
Due to the topography of the deposited layer of poly-2, having a significant difference between the height of the high and the low points of the deposited layer of poly-2, and the subsequent difficult to control etch of this layer, the etched layer of poly-2 will have a severe and negative impact on the quality of the LDD source/drain impurity implants.
It has been found that a major contributor to the latter negative impact is poly-2 residue that remains in place over sidewalls of the patterned layer of poly-1. This poly-2 residue is the direct cause of poor LDD source/drain implantations and of a low value of Isat of the subsequently created gate electrodes, using the patterned layers of poly-1 and poly-2.
In order to improve the above highlighted negative aspects of poor LDD impurity implantation and low Isat performance, the poly-2 residue must be removed from the sidewalls of the patterned poly-1.
It has in this respect been found that there is a direct correlation between the pattern density (P.D.) of the poly-1 and the residue of poly-2 that remains in place over sidewalls of the patterned poly-1. Higher poly-1 P.D. will result in increased formation of poly-2 residue over sidewalls of the patterned poly-1.
A layer 14 of poly-2 has been deposited over substrate 10, the (high) topography of layer 14 is apparent from the cross section shown in
After patterning of layer 14, the results of which are shown in the cross section of
As highlighted above, residues 16 further causes low Isat performance of the devices that are created using gate electrode 12.
A number of parameters are in effect during the etch of layer 14 of poly-2, the main parameters are essentially:
A number of experiments have been conducted, leading to the conclusions of the invention.
For the etching of the layer 14 of poly-2 a number of processing conditions are applied, similar for instance to the etching of organic polymer, whereby anisotropic etching is applied using a parallel HDP chamber, using a gas containing O2 at a flow rate of between about 10 and 40 sccm in a carrier gas of argon at a flow rate of between about 10 and 40 sccm. The flow rate of the carrier gas is adjusted to maintain a pressure of between about 1 and 10 mTorr in the reaction chamber, an rf power of between about 1,000 and 1,500 Watts is applied to the etching chamber.
The effect of varying the parameters that are conventionally applied for the etching of layer 14 has led to the conclusion that varying the participating parameters does not have an effect on the poly-2 residue over sidewalls of the patterned poly-1. For instance, varying source power from 250 Watts to 300 Watt during the OE step does not materially effect poly-2 residue deposition. Similarly, increasing the ME percentage of OE from 20% to 30% equally does not materially effect poly-2 residue deposition nor does increasing the EO time from 240 to 330 seconds.
Steps have been taken to reduce polymer formation and deposition, these steps also did not indicate any effect on the formation of poly-2 residue over sidewalls of the patterned poly-1.
The applied recipe has been varied in order to validate the impact of the native oxide effect on the formation of poly-2 residue, from this it has been concluded that poly-2 residue is not stimulated or induced by the presence of native oxide.
Further experiments have been directed at determining the effect of Pattern Density. From these experiments it has been concluded that poly-1 P.D. may have an impact on the formation of poly-2 residue. Poly-2 residue increases as the P.D. for poly-1 is increased.
These and other aspects of the invention are highlighted using the cross section of
The invention has solved the above highlighted problems, created by the formation of poly-2 residue over sidewalls of patterns layers of poly-1, by providing a new flash step and by making use of the following observations:
The processing conditions of the final etch-recipe for etching the layer 14 of poly-2, comprising steps BT-ME-OE-flash step, as provided by the invention is as follows:
The invention, which provides method for creation of stacked layers of polysilicon, can be summarized as follows:
Alternately, the invention, which provides for the stacked layers of polysilicon for creation of mixed-mode semiconductor devices, can be summarized as follows:
The cross sections that are shown in
The cross section that is shown in
As seen in
The surface of the floating gate 200 has been highlighted as surface 260. Surface 260 typically is a layer of poly-oxide that has been formed over the surface of layer 200 of floating gate material by oxidizing the layer of semiconductor material 200, such as polysilicon, that has been deposited for the creation of the floating gate 200, using methods of photolithography and exposure to expose layer 200 over surface area 260.
In the structure shown in
To program the transistor shown in
The programming and erasing of an EEPROM cell is accomplished electrically by using the familiar Fowler-Nordboirn (F-N) tunneling effect. During programming, a sufficiently high voltage is applied go to the control gate 240 and the drain 140 while the source 120 is grounded, creating a flow of electrons in the channel region 160 in the substrate 10. Some of these electrons gain enough energy to transfer from the substrate 10 to the floating gate 200 through the thin gate oxide layer 180 by means of Fowler-Nordheim tunneling. As the electron charge increases on the floating gate 200, the electric field between the control gate 240 and the drain 140 is reduced, which reduces the electron flow. Of importance in the tunneling region 300 and 320 is the quality and the thinness of the tunneling oxide 180 separating the floating gate 200 from the substrate 10. Inadvertent reverse tunneling, or erasure, for example, may occur if the tunnel oxide 180 is degraded, or other barriers to reverse tunneling are not formed in a split-gate flash memory cell.
A sidewall spacer (not shown) is typically provided over the exposed surface of the control gate 240 where this control gate does not overly the floating gate 200.
For the creation of the split-gate flash memory cell that is shown in cross section in
The floating gate 200 is next covered with a layer 220 of inter-poly dielectric which forms the separation between the floating gate 200 and a thereover created control gate 240. For this layer 220 of spacer dielectric, a layer of Oxide/Nitride/Oxide (ONO) is frequently applied. The control gate 240, of a second conductivity type, is then formed overlying the layer 220 of spacer dielectric.
Where the cross section of
The cross section of
Operational details and details of creation of the structure that is shown in cross section in
Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.